Electrochemical machining electrolyte

ABSTRACT

Hygroscopic additives for sodium and potassium chlorate and perchlorate electrochemical machining electrolytes. These additives have a fire retardant effect on combustibles exposed to electrolyte splash. When used in conjunction with relative humidities of above about 35 percent, the additives prevent the electrolyte splash from drying sufficiently to support combustion. Specific materials which are compatible with the electrolyte are disclosed.

United States Patent [19] Darling et al. 1 Feb. 27, 1973 ELECTROCHEMICAL MACHININ [56] Relerences Cited ELECTROLYTE UNITED STATES PATENTS [75] Inventors: Jacob B. Darling, Carmel, Ind.;

Mitchell LaBoda Detroit, Mich 3,616,344 10/1971 Peterson et a], ..204/143 M 3,274,105 9/1966 Mevel ..252/2 [73] Assignee: General Motors Corporation,

Detroit, Mich. Primary Examiner-John H. Mack Assistant Examiner-Regan J. Fay [22] Flled' 1971 Attorney-Paul J. Ethington et al. [21] Appl. No.: 117,693

[57] ABSTRACT Related [1.8. Application Data Hygroscopic additives for sodium and potassium DlvlslOnOfSeLN-750772,A"8-7,1968- chlorate and perchlorate electrochemical machining electrolytes. These additives have a tire retardant ef- U.S. feet on cgmbustibles exposed to electrolyte plash When used in conjunction with relative humidities of [51] Int. Cl. ..B23p l/00 above about 35 percent the additives prevent the electrolyte splash from drying sufficiently to support g H I combustion. Specific materials which are compatible [58] Field of Search..252/62.2, 8.1, 187; 204/143 M, with the electrolyte are disclosed.

ELECTROCHEMICAL MACHINING ELECTROLYTE This application is a division of application Ser. No. 750,772 entitled ELECTROCHEMICAL MACHIN- ING ELECTROLYTE", filed August 7, 1968, in the names of Jacob B. Darling and Mitchell A. LaBoda, and assigned to the assignee of this application.

This invention relates generally to electrochemical machining electrolytes and more specifically toward alkali metal chlorate and/or perchlorate based electrolytes and fire retardant additives therefor.

This invention is an improvement to the electrolyte disclosed and claimed in copending U.S. Pat. application Ser. No. 664,770 now abandoned entitled Electrochemical Machining Electrolyte filed on Aug. 31, 1967, in the name of Mitchell A. LaBoda and assigned to the assignee of the incident invention. It is intended that the aforesaid patent application be incorporated herein by reference. The aforesaid copending application relates to alkali metal chlorate and perchlorate based electrolytes. These electrolytes, hereafter referred to simply as chlorate electrolytes, have revolutionized the electrochemical machining industry by providing a unique combination of properties which permit the rapid and accurate machining of metals with little or no overcut or splash cutting while at the same time producing machined surfaces having finishes as low as 2-5 microinches.

Although the use of these chlorate and perchlorate based electrolytes have recently taken on considerable commercial importance in the electrochemical machining industry, it is not without concern over a particular disadvantage associated with their use. This disadvantage is the degree of care with which these electrolytes should be handled and used. When in solution, the chlorate and perchlorate salts do not present a problem, per se. However, when dry and in contact with organic combustible materials, such as paper, cloth or wood, the possibility of combustion arises. As a result, the electrolyte has heretofore been used under closely controlled conditions. Special clothing is often worn in the work area. Speciai handling of the clothing used in the work area was necessitated. Other precautionary measures have also been taken. While the precautions taken in conjunction with the use of chlorate and/or perchlorate electrolytes are no more stringent than for many other industrially used chemicals, it would, nonetheless, be desirable to eliminate, as much as possible, the need for such precautions. The elimination or substantial reduction of the need for such precaution contributes to the economic benefits of the use of the chlorate and/or perchlorate electrolytes.

Preferably it is desirable to merely add something to the electrolyte which would reduce the possibility of fire. However, the chlorate and perchlorate electrolytes are somewhat sensitive to the addition of high concentrations of many foreign ions, particularly anions. For example, more than about a 5% addition of chloride ion bearing compounds have a detrimental affect on the low throwing characteristic of the chlorate electrolytes so that wild cutting becomes increasingly apparent and dimensional control is more difficult to hold. On the other hand, a 5 percent chloride ion addition does not appreciably affect the surface finish or cutting rate characteristics of the chlorate electrolytes.

Other foreign materials tend to affect one or more of the three principal characteristic properties of the chlorate electrolyte.

We have now found that certain materials can be added to chlorate electrolytes to reduce the likelihood of fire and which materials are compatible with the electrochemical machining characteristics of these electrolytes.

It is, therefore, an object of this invention to reduce the likelihood of combustion which might otherwise occur incident to the inadvertent or accidental splashing and subsequent drying of chlorate electrolytes on combustibles and to do so by adding to the electrolyte hygroscopic compounds which are substantially compatible with the unique electrochemical machining characteristics of chlorate electrolytes.

This and other objects and benefits of the incident invention will become more apparent from the detailed disclosure which follows.

Briefly stated, this invention relates to the addition of ECM-compatible hydroscopic materials to chlorate electro-chemical machining electrolytes. By ECM- compatible materials are meant materials which when added to chlorate ECM electrolytes do not materially interfere with any of the three principal ECM characteristics for which the chlorate electrolytes are unique. When used in conjunction with relative humidities controlled to be in excess of about 35 percent, the hygroscopic materials will retain or absorb sufficient moisture from the ambient air to preclude the splash from drying to a degree sufiicient for combustion. When the relative humidity is in excess of about 72 percent, the chlorate and perchlorate salts are sufficiently hygroscopic themselves to prevent drying. For relative humidities below about 72 percent, but in excess of about 59 percent, lithium bromide, potassium iodide, magnesium perchlorate, lithium citrate, potassium lactate and potassium acetate may be used in concentrations of less than about 50 g/l. When the relative humidity is as low as about 35 percent, lithium bromide and potassium acetate have been found to be most effective.

As disclosed and claimed in the aforesaid U.S. Pat. application Ser. No. 664,770, chlorate electrochemical machining electrolytes have effectively been used at concentrations of about 50 g/l of the salt upwards to about the saturation point of the salt in the solution. The most practical operating concentration range of the chlorate appears to be about 350-500 g/l. As indicated heretofore, under certain conditions dried chlorate electrolyte in contact with combustible organic matter increases the likelihood or risk of fire. In this regard, generally it is noted that the higher the concentration of the chlorate salt in the electrolyte, the less chance there is fire. This is so because the chlorate salt is hygroscopic and will itself absorb sufficient moisture from the atmosphere to prevent ignition and/or retard burning rates. This is true even at moderate relative humidities. It has also been noted that at relative humidities in excess of about -72 percent, even the lower concentration chlorate solutions are not a significant fire hazard, because the electrolyte splash will not dry sufficiently to cause a fire. By this invention this fire immunity is extended to chlorate electrolytes having low concentrations of chlorates (e.g., less than about 500 g/l) and which are used in environments having relative humidities less than about 72 percent. It has been found that certain hygroscopic materials may be added to the chlorate electrolytes without being materially detrimental to the principal ECM characteristics of the chlorate electrolytes. These materials give the chlorate electrolytes the desired fire immunity. It is noted that no salts have been found which are perfectly ECM compatible with the chlorate electrolytes. Many have been found which are incompatible for one reason or another. Hence the truly effective additives involve a compromise of benefits. In this regard, for example, we have noted generally that salts such as lithium bromides and potassium iodide have virtually no affect on the cutting rate or the surface finish benefits obtainable from the chlorate electrolytes. However, when used in concentrations above about 50 g/l, lithium bromide and lithium iodide tend to affect the accuracy with which a piece may be machined by reducing somewhat the low throwing power characteristic of the electrolyte which in turn gives rise to some uncontrolled or wild cutting. When these salts are used in concentrations below about 50 g/l, the affects on the low throwing power characteristic might well be tolerated in view of the fire immunity benefits obtainable by their addition. Other additives have other affects. For example, salts such as lithium citrate, potassium lactate and potassium acetate have been found to slow down the cutting rate of the chlorate electrolytes but seem to have no affect on the surface finish and/or wild cutting characteristics of that electrolyte. Generally speaking, it appears that the inorganic salts tend to affect only the low throwing power characteristic, and the organic salts tend to affect only the cutting rate characteristic of chlorate electrolytes.

A particularly interesting compound for purposes of this invention is magnesium perchlorate if used in concentrationsof about 37 g/l or more, and in an environment of at' least about 60 percent relative humidity (i.e., about 60 percent or more). It is very effective as a fire retardant and additionally provides the benefit of being the most nearly ECM compatible of all the additives tested, apparently owing to the commonality of the anions involved. Magnesium perchlorate is not recommended for use in environments wherein the relative humidity is appreciably lower than the aforementioned 60 percent relative humidity. This is so because it has been noted that at lower relative humidities the magnesium perchlorate tends to accelerate the burning rate rather than decrease the likelihood of fire.

As shown in Table l, which follows, each additive has its own concentration range requirements for providing acceptable results. With the exception of magnesium perchlorate, the acceptable concentration range appears to be between about g/l to about 50 g/l. At concentrations below the minimum value little or no fire retardant benefits are observed. Generally at concentrations above about 50 g/l, the ECM compatibility deteriorates rapidly to the point where many of the chlorate electrolyte advantages are lost. Magnesium perchlorate, on the other hand, should have at least about 37 g/l for effective fire retardancy and has no practical discernible upper limit owing to compatibility of the anions. On the other hand, because of the 60 percent relative humidity requirement, magnesium perchlorate lacks the quality of being universally useful over a broad range of humidities.

Table l reflects some preferred additives and the concentrations found to be useful in accordance with this invention.

TABLE I Additive 59% 63% RH 35% RH Pref- Pref- Range g/l ferred g/l Range g/l erred g/l UB1 5 25 7.5 10 25 50 about 37 K! 15 25 15 Mg(C1O 37 LiC.H,O 5 15 5 KC,H,O, 25 50 37 42 25 50 about 37 KC,H,O, 5 5 3 l5 25 Relative humidity Table 11 reflects the results of a series of tests on paper to determine the fire retardancy benefits of certain salts within our invention when used at relative humidities of about 59-63 percent.

TABLE I1 2lbs./gal Chlorate 31bs./gal Chlorate Impact Burning Impact Burning Rate Rule Sensiln/Sec. Sensi ln/Sec. tive -tivc Untreated No 1.14 No 1.14 NsClO, (only) Yes 2.11 Yes 1.91 LiBr 5 g/l Yes 0.71 Yes 1.09- 1.15 15 g/l No 0.12 No 0 25 5/1 No 0 No 0 so 5/! No 0 N0 0 K1 5 g/l No 1.144) Yes 0.75 15g/l No 016-0 No 1.06 25 g/l No 0.16-0 No 014-0 50 g/] No 0 No 009-0 3 5 4):

5 g/l No 0 No 0 15 g/l No 0 No 0 25 g/l No 0 No 0 50 g/l No 0 No 0 LiCJfiO, 5 g/l No 0 No 0.80-0 15 g/l No 0 No 0 25 g/l No 0 No 0 50 g/] No 0 No 0 s s 5 g/l Yes 1.20 Yes 0.86 15 g/l Yes 1.50 Yes 1.38 25 g/l Yes 1.50 Yes 1.24 g/l Yes 0.84 Yes 0.80

KC,H,0, 5.3 g/l No 0 Yes 1.20 16.0 g/] No 0 Yes 1.34 26.7 /1 No 0 Yes 1.09 53.0 g/l No 0 Table III reflects the results of a series of tests on paper to determine the fire retardancy benefits of certain salts within our invention when used at relative hu midities of about 35-38 percent.

TABLE III 2lbs./ga1 NaCIO, 311m lgul (.IU, Impact Burning lmpmzl Hunting Rate Rain Semi ln/Sec. Semi ln/Sec -t|ve -tive Untreated No 1.14 No 1.14 NaClO, (only) Yes 2.27 Yes 2.27 LiBr 53/! Yes 2.12 Yes 1.11 15 g/l Yes 0.95 Yes 1.20 25 3]! Yes 0.51 Yes 1.38 50 g/l N0 0.1-0 No 0.02 KC,H,O,

g/l Yes 1.29 Yes 1.17 15 g/l Yes 1.29 Yes 1.33 25 g/l Yes 0.79 Yes 1.56 50 gll Yes 0.45 Yes 0.86

Table IV reflects the results of a series of tests on V z H3 3 es es shop cloths to determlne the fire retardancy benefits of 25 8/! yes 059 Yes certam salts within our tnventlon when used at relative 50 g]! No 0.09 No 0.01

KC,H,O, hum1d1t1es of about 59 63 percent. 5 S B A Yes 109 Yes 13' I Yes 0.40 Yes 0.90 TABLE Iv 25 3/] Yes 0.41 Yes 0.66 2 lbs/gal NaClO, 3 lbs/gal ClO, 50 gll N 0.08 Yes 0.24 lmpact Burning lmpact Burning Rate Rate sensitive In/Sm sensitive Ms Table Yl reflects the ECM compatiblllty of certain Untreated No 0.28 No 0.28 salts w1th1n our 1nvent1on.

2#/gal. NaClOa Bit/gal. NaClOa 12 V0115 Volts 12 Volts 10 Volts A E c A B c A a IB 1 Std. sample 0.102 E E 0.117 E E 0.102 E E 0.128 E E LlBr:

5 0. 084 E E 0.103 E E 0. 007 E E 0.124 E E I 15 0.070 E E 0.101 E E 0.100 E E 0130 E E W 211 0. 089 (1 E 0.114 (1 E 01011 (1 E 0.125 (1 E 50 0. 000 (1 E 0.110 (1 E 01011 11 E 0.124 (1 E Kl:

0 0.100 E E 0.12:1 11 E 0.11:1 E E 0.130 11 E 1.1 0.101 11 E 0.124 11 E 0.100 11 E 0. 1:111 11 E 0.1115 11 E 01:10 1 E 0.110 11 E 01:17 1 E 50 0.1011 11 E 0.12.1 11 E 0.114 11 E 0141 11 1 I1i('11|l5()1:

' 5 g 0. 0110 E E 0. 005 E E 0.002 E E 0.12:1 E 1 15 0. 002 E E 0. 002 E E 0.101 11 E 0.11111 E 1 .11. 2s 0.070 E E 0.088 E E 0. 012 E E 0.000 E 1 50 0. 000 11 E 0. 000 E E 0. 070 11 E 0. 0011 E 1 100 0. 005 11 E 0. 008 E E Kcgnscgi a 0.080 E E 0. 020 E E 0.000 E E 0.100 E E 15 0.070 E E 0.088 E E 0.077 E E 0.115 E E 25 0.000 E E 0.002 E E 0017 E E 0.100 E E 50 0.000 E E 0.082 E E 0.004 (1 E 0. 0011 11 E KC3H5O1 5.0 0. 000 E E 0.111 E E 0. 000 E E 0.122 E E 10.0 0. 005 E E 0.112 E E 0.002 E E 0.111 E E F -{20.7 0.075 E E 0.100 E E 0.077 E E 0.107 E E 50.0 0.070 E E 0. 011:1 E E Legend: A=M0t11l romovnl 111t11l11./n ln. B= 110k 01' wild cutting. =S|1r[111-1 1i11lsl1. Ratings: E=Exc0ll011t. (l==(loml. F=F11i1z l= l'onr.

21 1011 1111) Yes Yes 129 For purposes of these tests, the preferred standard of No 0 No 0 acceptability for a fire retardant additive was whether 15 g'! No 0 No 0 that additive would produce a burning rate which was :3 :2 8 :8 8 approximately equal to or less than the burning rate of K1 40 the untreated test sam le while at the same time be I u p a fig g 3- compatible w1th the ECM character1st1cs of the 25 g/l No 0 N0 0 chlorate electrolytes. A less rigid, but nonetheless acggsg N0 0 N0 0 ceptable, standard is whether the particular additive 5 g Yes 120 No 0 would reduce the burning rate of the test sample below 15 g/l Yes 1.00 No 0 that of samples treated only with NaClO electrolytes :3 g 8' 3 without additives. Lic,11,o, The data reported in the tables was derived from exa E3 8 :2 8- periments briefly described hereafter. Two commonly 25 8/1 No 0 N0 0 found organic materials (i.e., paper toweling and shop a ga /i 0 N0 0 N0 0 cloths) were selected as the test materials. Untreated 5 J Yes H7 Yes H1 towels and shop cloths were used as the first standard. 15 gll Yes 0.57 Yes 0.50 Towels and shop cloths treated in electrolytes which is 5% :2: 8'3; 3:: 3'3; did not have the hygroscopic additives were used as the -1 0 5 5 second standard. All other towels and shop cloths were g g g S3 3 3 6-1; treated in various solutions of electrolytes containing g" No 0 No different additives at varying concentration levels. 5310 8/ N0 0 Stock solutions of sod1um chlorate contaming about Table V reflects the results of a series of tests on shop cloths to determine the fire retardancy benefits of certain salts within our invention when used at relative humidities of about 35-38 percent.

TABLE V 2 lbs/gal NaClO 3 lbs/gal NaClO lmpact Burning Impact Bumlng Rate Rate sensitive ln/Sec. Sensi ln/Sec.

-tive Untreated No 0.28 No 0.28 lItI aBCIO, (only) Yes 2.27 Yes 2.56

2 lbs/gal (230 g/l) and about 3 lbs/gal (350 g/l), were used for all experiments. Additions of the additives to be investigated were made to one liter portions of the stock solutions. All samples to be tested were equilibrated at the same relative humidity range being investigated. 2 in. X l4 in. strips of treated materials were cut from paper toweling and the shop cloth. The paper toweling was identified as Scott C-Fold, Brand 150, ultra-high absorbency, white. The shop cloth selected was identified as KEX" National Service. The strips were soaked in the electrolytes to be tested and were hung out to drip dry for about four hours in the laboratory which had a relative humidity of about 40 percent. After this drip dry period, the strips were equilibrated in a controlled humidity box for a period of at least '24 hrs. The controlled humidity box was a polyethylene tank 18 inches X 18 inches X 24 inches fitted with a sealing lid. The tank was made air-tight by means of sealing around the edges with tape. Air movement within the box was effected by a motor-propeller arrangement. A relative humidity recorder was used inside the chamber to monitor both the relative humidity and the temperature within an accuracy of about i 2 percent relative humidity and about t 1F. The humidity in the box was controlled by use of combinations of salts in contact with their saturated solutions as described in the Handbook of Chemistry and Physics, page 2309, Chemical Rubber Publishing Co., Cleveland, Ohio (1953-1954). The salt used in the 59-63 percent relative humidity range and in the 72 percent relative humidity experiments was sodium bisulphate. In the 35-38 percent relative humidity range chromic acid anhydride was used in the controlled humidity box. The burning rate tests were conducted in a steel box made to hold a sample rack in a vertical position. A Bunson Burner flame, 1% inch high was used to ignite the samples; ignition time was 1 sec. for the paper samples, and 5 secs. for the cloth samples. 12 inch portions of the samples were ignited in the burning chamber. Burning time was measured and a burning rate calculated.

In addition to the burning rate tests, impact tests were also conducted. For the impact sensitivity test, 2 inch squares were cut from the ends of the paper and cloth strips tested in the burning rack. The squares were placed on a tile and manually struck several times with a ball peen hammer swung through an arc of about 12 inches.

As indicated before, the preferred materials which are considered to be most useful are those which had burning rates equal to or less than the ordinary, untreated samples and would produce these low rates at concentrations low enough so as not to materially interfere with the ECM characteristics defined above. In this regard, for example, it was found that several material: were effective for fire retardancy, but required higher concentrations than thought to be compatible with the ECM characteristics of the chlorate solution.

The ECM compatibility tests were conducted using a 5160-]! steel tube ground electrochemically across its face for l min. using a gravity-feed system. The tube was ground usiia cd ieiayfie aluminum oxide wheel made by Bay State Abrasive Products Company of West Borough, Massachusetts. Tests were conducted using both 12 and 16 volts. The tube lengths were measured with a micrometer before and after electrochemical machining to determine the metal removal rate. The tube ends were next subjectively compared under a binocular microscope at 20 magnifications. The basis for this subjective comparison was two-fold, the first being a change in the reflectivity of the surface, and the other being a change in the tube-end profile, the latter being an indication of any wild-cutting activity.

Breakdown of some of the hygroscopic additive materials was observed in a number of cases when the electrolyte was subjected to electrochemical machining conditions. Accordingly, periodic additions of the hygroscopic agent may be necessary to hold the concentration of the additive at an effective level.

While our invention has been described solely in terms of certain specific embodiments thereof, we do not intend to be limited thereto but rather only to the extent defined hereafter.

We claim:

1. An aqueous electrochemical machining electrolyte for use in an environment having a relative humidity of about 59 percent or more, said electrolyte consisting essentially of a salt selected from a group consisting of sodium chlorate and potassium chlorate and at least one ECM-compatible ignition-suppressant selected from the group consisting of at least about 5 grams per liter and less than about 25 grams per liter lithium bromide, at least about 15 grams per liter and less than about 25 grams per liter potassium iodide, at least about 37 grams per liter magnesium perchlorate, at least about 5 grams per liter and less than about 15 grams per liter lithium citrate, at least about 5 grams per liter and less than about 53 grams per liter potassium lactate and at least about 25 grams per liter and less than about 50 grams per liter potassium acetate.

2. An aqueous electrochemical machining electrolyte for use in an environment having a relative humidity of about 35 percent or more, said electrolyte consisting essentially of at least one salt selected from the group consisting of sodium chlorate and potassium chlorate and at least about 25 grams per liter and less than about 50 grams per liter of at least one ECM-compatible ignition-suppressant selected from the group consisting of lithium bromide and potassium acetate. 

2. An aqueous electrochemical machining electrolyte for use in an environment having a relative humidity of about 35 percent or more, said electrolyte consisting essentially of at least one salt selected from the group consisting of sodium chlorate and potassium chlorate and at least about 25 grams per liter and less than about 50 grams per liter of at least one ECM-compatible ignition-suppressant selected from the group consisting of lithium bromide and potassium acetate. 